1. Field of the Invention
The present invention relates to a composite magnetic sheet preferable for a magnetic part for a coil, and a production method thereof.
2. Description of the Related Art
Recently, with the trend of a thinner shape, a smaller size and a higher density implementation of electronic appliances, or a substrate structure of a multiple layer arrangement, or the like, a smaller size, a lower height and a thinner shape of magnetic parts such as coil parts to be mounted on the electronic appliances are more and more required. Additionally, to the coil parts used for a power source circuit, a higher performance concerning the electric characteristics for dealing with a high electric current, or the like and cost reduction of the magnetic parts are also highly required.
Conventionally, the magnetic parts used for the coil parts have been produced for example by a method as follows. First, a paste is prepared by mixing ferrite magnetic substance powders with a binder, a solvent, or the like for forming a thin magnetic sheet of a 10 to 100 μm thickness. Then, the magnetic sheet is provided with a conductor line path or a connecting electrode on its surface and a plurality of resultant sheets are laminated. A laminated body of the magnetic sheets is pressed with a pressing machine as a magnetic member for the coil parts and then baked integrally in a baking furnace. Thereafter, an external electrode is formed on a side end face of the sintered compact for providing a laminated chip coil (see, for example, Japanese Patent Application Laid-Open No. 1994-333743, paragraph number 0010, FIG. 1).
Moreover, a production method as follows is also employed. Powders prepared by covering the surface of flat metal magnetic powders with an insulating layer are added to a resin material so as to be mixed and dispersed sufficiently in an organic solvent. Subsequently, a coating film is formed by applying the above-mentioned slurry onto a supporting member. Thereafter, by carrying out the in-plane alignment process by adjusting the magnetic field intensity, the coating film is dried so as to produce a composite magnetic sheet (see, for example, Japanese Patent Application Laid-Open No. 2004-247663, paragraph numbers 0009 to 0036, FIG. 1).
However, the above-mentioned conventional magnetic parts involve the following problems. The magnetic sheet disclosed in Japanese Patent Application Laid-Open No. 1994-333743 is preferable for a part of a low-height-type laminated chip coil. However, the magnetic sheet is produced by wet mixing of magnetic substance powders mainly made of a Ni—Cu—Zn based ferrite, or the like, and a binder made of a mixture of a resin and an organic solvent, or the like, and forming a coating film of the obtained slurry and executing a drying process. Furthermore, since a sintering process is essential in order to obtain preferable magnetic characteristics as a magnetic core for a coil part, the production processes and the time are prolonged and the cost is raised, and thus it is problematic.
Moreover, the composite magnetic sheet disclosed in Japanese Patent Application Laid-Open No. 2004-247663 is suitable for an inductance element to be mounted on a printed wiring board. However, since the composite magnetic sheet is also produced by wet mixing of magnetic substance powders, and a binder of a mixture of a resin and an organic solvent, or the like, and forming a coating film of the obtained slurry and executing a drying process, the substantial production process of the magnetic sheet, the conditions, the elements, or the like required therefor are same as those of Japanese Patent Application Laid-Open No. 1994-333743 so that the problems of the time and the cost rise remain. Moreover, since the effective magnetic permeability μ of a composite magnetic sheet tends to be low due to the factors such as decline of the effective filling ratio of the magnetic substance powders derived from voids generated by evaporation of the solvent at the time of drying the composite magnetic sheet after coating, and the low magnetic permeability μ in general of the metal based magnetic substance powders used mainly for the composite magnetic sheet compared with the ferrite based magnetic substance powders, the filling ratio of the magnetic substance powders in the composite magnetic sheet needs to be increased by using metal based magnetic substance powders of a flat shape, including a large amount of metal based magnetic substance powders as much as possible, or the like. However, in the case that the addition amount of a resin and a solvent, which provides the flexibility and the bonding property, is reduced, since the vulnerability of the magnetic sheet becomes remarkable, the amount of the metal magnetic substance powders to be added has its limit so that an obtainable magnetic permeability is limited as well.